Blackspot is a physiological (non-infectious) disorder affecting potato tubers damaged during handling. It is also known as blue discoloration, blue spotting, bluing, bruise, internal blackspot, internal bruising, internal grayspot and stem-end blackening. The disorder appears as an internal discoloration and blackening that can be seen when injured tubers are peeled or sliced. The blackening is usually restricted to the outer 1/4" to 1/2" of tuber tissue between the skin and the vascular ring. The color of the spot can vary from a light gray to a blue-gray to an intense coal black. The size and intensity of the spot usually reaches a maximum within 24 hours of bruising and once formed, the blackened area will not disappear.
Blackspot is normally caused by impacts (bumping, dropping, etc.) to the tubers during handling, transportation, storage or packaging but may also be associated to varying degrees with other physical damage such as pressure bruising and/or shatter cracking. The force required to initiate a blackspot need not be severe, particularly to tubers of susceptible cultivars.
The disorder was first reported and described in England in 1912 and since then it has become a serious problem throughout Europe. It was identified in the United States in 1940 and can now be found in all the potato growing areas in this country as well.
Although affected potatoes can be eaten, they are of limited commercial value because of their appearance. The disorder is particularly serious because affected tubers may show no external damage, even after the tuber is washed. Blackspot bruise often results in serious economic losses in both fresh market and processed potatoes, including chips and fries.
Tuber susceptibility and an impact of sufficient magnitude to rupture cells are the two most important factors responsible for initiation and development of blackspot. These conditions activate a series of four biochemical conversions of phenolic compounds (beginning with tyrosine) to conjugated quinones. Intermediate compounds include caffeic acid and p-coumaric acid. This sequence, which is mediated by the action of polyphenyloxidase enzymes, is followed by the polymerization of the quinones to the black pigment melanin. In healthy, non-damaged tissue these phenolic compounds and the polyphenyloxidase enzymes are normally compartmentalized separately and do not come into contact. However, cell rupture causes the contents to mix and the blackening reaction occurs. Although tyrosine and polyphenyloxidase enzymes play a major role in the development of blackspot, the total amount of these compounds present in tubers usually does not correlate with tuber susceptibility or explain differences in blackspot reaction between tubers and cultivars. The most recent work on the biochemistry of blackspot phenomenon indicates that reduction of the free tyrosine pool within the cell increases tuber resistance to blackening. (Corsini et al., Evidence for highly conserved tuber tyrosine levels among potato genotypes and implications for blackspot resistance. Am. Potato J. 66:511-512 (1989)). These investigators found total tyrosine content of many potato cultivars to be remarkably similar though these cultivars ranged widely in their susceptibilities to blackspot. Those cultivars with the greatest resistance to blackspot had a large proportion of the tyrosine bound into protein with very little free tyrosine available for melanin formation. The opposite was found to be true in the susceptible cultivars.
In addition to the biochemical factors discussed above, there are numerous environmental and cultural factors that can contribute to the manifestation of this disorder. Tuber turgor pressure, temperature, specific gravity, mineral nutrition, date of planting, soil moisture, and soil temperature can all influence blackspot development (Hiller et al., Physiological disorders of potato tubers. Potato Physiology 389-455, Academic Press, New York (1985)).
Even when all the predisposing factors are considered, potato cultivars vary markedly in their response to impact damage. Some cultivars may be highly resistant to blackspot while others may be highly susceptible. Tubers from a single plant may differ in their blackening responses. Susceptibility may also vary from the stem end to bud end of an individual tuber.
Losses due to blackspot can be managed to a certain extent by production practices employed during the growing season. Practices currently employed to control blackspot bruising are to keep the plants as healthy as possible by providing adequate disease and pest control, and good soil moisture and soil fertility (particularly potassium). Soil should not be allowed to dry out prior to harvest and vines should be killed early to reduce water loss from the tubers. The most important means of controlling the extent of blackspot formation is reducing tuber injury both during and after harvest. This can be implemented by not harvesting tubers when the soil temperature is low (8.degree. C.) and by adjusting operation speeds, drop lengths and padding on all potato handling equipment. Selection of cultivars that are more resistant to blackspot is also an important consideration. However, this is not always possible because of production restrictions. Some cultivars demonstrate great potential for commercial production but suffer because of blackspot bruising. Such is the case with the cultivar Lemhi Russet and to a lesser extent, Russet Burbank.
`Lemhi` and Russet Burbank have many characteristics that make them suitable and important cultivars for the processing industry, including low reducing sugar levels, good storability, excellent processing and dormancy. However, they are extremely susceptible to blackspot. Naturally resistant Lemhi and Russet Burbank have not been found. This limits their acceptability because the quality of processed products (e.g. chips and fries) obtained from bruise-damaged potatoes is lowered substantially. Although resistance to blackspot may be attainable through traditional breeding techniques, other characters which make the cultivar commercially acceptable, such as sugar levels, shape, specific gravity, or yield, could also change. It is difficult to maintain every desirable character while breeding for cultivar improvement and such an approach would involve several years of crossing and testing.
As an alternative to traditional breeding techniques, plant cell culture provides the opportunity to evaluate large quantities of cells (literally millions), having the potential of regeneration into valuable somaclonal variants. Normally, a large population of regenerated plants is required in order to identify somaclones with the desired traits. Increasing and testing such populations is labor intensive and requires a tremendous amount of greenhouse and field space. This problem is usually addressed by developing techniques that will allow the somaclones to be screened for the required characteristic(s) while in tissue culture, prior to being regenerated into plants. Evaluation at the cell culture level greatly reduces space involved and increases the number of somaclonal lines that can be examined. In vitro screening procedures essentially increase the likelihood of identifying clones with desirable traits by eliminating unwanted material. Accordingly, there is a need to provide an alternative to a traditional breeding approach for potato cultivar improvement and development. There is also a need for increasing the likelihood of identifying blackspot resistant `Lemhi` and Russet Burbank potato clones. There is further a need for techniques which increase the ease and efficiency of identifying and selecting prospective blackspot resistant `Lemhi` and Russet Burbank somaclones. The present invention fulfills these needs and provides other related advantages.